The conversion of oxygenates to olefins (OTO) is currently the subject of intense research because it has the potential for replacing the long-standing steam cracking technology that is today the industry-standard for producing world scale quantities of ethylene and propylene. The very large volumes involved suggest that substantial economic incentives exist for alternate technologies that can deliver high throughputs of light olefins in a cost efficient manner. Whereas steam cracking relies on non-selective thermal reactions of naphtha range hydrocarbons at very high temperatures, OTO exploits catalytic and micro-architectural properties of acidic molecular sieves under milder temperature conditions to produce high yields of ethylene and propylene from methanol.
Current understanding of the OTO reactions suggests a complex sequence in which three major steps can be identified: (1) an induction period leading to the formation of an active carbon pool (alkyl-aromatics), (2) alkylation-dealkylation reactions of these active intermediates leading to products, and (3) a gradual build-up of condensed ring aromatics. OTO is therefore an inherently transient chemical transformation in which the catalyst is in a scontinuous state of change. The ability of the catalyst to maintain high olefin yields for prolonged periods of time relies on a delicate balance between the relative rates at which the above processes take place. The formation of coke-like molecules is of singular importance because their accumulation interferes with the desired reaction sequence in a number of ways. In particular, coke renders the carbon pool inactive, lowers the rates of diffusion of reactants and products, increases the potential for undesired secondary reactions and limits catalyst life.
Over the last two decades, many catalytic materials have been identified as being useful for carrying out the OTO reactions. Crystalline molecular sieves are the preferred catalysts today because they simultaneously address the acidity and morphological requirements for the reactions. Particularly preferred materials are small pore size (diameter less than or equal to about 5 Angstroms) molecular sieves, especially those having pores defined by eight-membered ring channel systems, such as those having the chabazite (CHA) framework type.
CHA framework type molecular sieves appear to be particularly suitable catalysts for the OTO reaction since they have cages that are sufficiently large to accommodate aromatic intermediates while still allowing the diffusional transport of reactants and products into and out of the crystals through regularly interconnected window apertures. By complementing such morphological characteristics with appropriate levels of acid strength and acid density, working catalysts are produced. Extensive research in this area indicates that in the case of CHA framework type aluminosilicates, increasing the silica to alumina molar ratio of the molecular sieve seems to be a key requirement in its use in OTO reactions.
Chabazite is a naturally occurring zeolite with the approximate formula Ca6Al12Si24O72. Three synthetic forms of chabazite are described in the following references: Zeolite K-G, described in “The Hydrothermal Chemistry of the Silicates. Part VII: Synthetic Potassium Aluminosilicates,” J. Chem. Society (1956), pages 2882-2891, Barrer et al; Zeolite D, described in British Patent No. 868,846 (1961); and Zeolite R, described in U.S. Pat. No. 3,030,181 (1962). Zeolite K-G zeolite has a silica:alumina mole ratio of 2.3:1 to 4.15:1, whereas zeolites D and R have silica:alumina mole ratios of 4.5:1 to 4.9:1 and 3.45:1 to 3.65:1, respectively. The relatively low silica to alumina molar ratio of these materials makes them less than optimal as catalysts for OTO reactions.
Considerable work has therefore been conducted on the synthesis of CHA framework type aluminosilicate molecular sieves having high silica to alumina molar ratios and in particular with silica to alumina molar ratios greater than 15:1, preferably greater than 100:1.
For example, U.S. Pat. No. 4,544,538 describes the synthesis of a synthetic form of chabazite-type aluminosilicate, SSZ-13, using N-alkyl-3-quinuclidinol, N,N,N-tri-alkyl-1-adamantylammonium cations and/or N,N,N-trialkyl-exoaminonorbornane as a directing agent in a conventional OH− medium. According to the '538 patent, SSZ-13 typically has a silica to alumina molar ratio of 8 to 50 but it is stated that higher molar ratios can be obtained by varying the relative ratios of the reactants in the synthesis mixture and/or by treating the zeolite with chelating agents or acids to remove aluminum from the zeolite lattice. However, attempts to synthesize SSZ-13 in OH− media at silica to alumina molar ratios in excess of 100 have been unsuccessful and have produced ITQ-1 or SSZ-23, depending on the alkali metal cation present. Moreover, increasing the silica to alumina molar ratio of SSZ-13 by dealumination has met limited success.
U.S. Pat. No. 6,709,644 describes a zeolite that is identified as SSZ-62 and has a CHA framework-type and a crystal size of 0.5 micron or less. SSZ-62 is said to have a silica to alumina molar ratio in excess of 10, such as in excess of 30, but the only synthesis example produces a material with a silica to alumina molar ratio of 22. Synthesis is effected in a hydroxyl medium in the presence of N,N,N-trimethyl-1-adamantammonium cation as the structure directing agent. The zeolite can be steamed, purportedly to help stabilize the crystalline lattice to attack from acids.
An all silica crystalline molecular sieve having the CHA framework type has been hydrothermally synthesized using N,N,N-trimethyladamantylammonium in hydroxide form as the structure-directing agent, but the synthesis requires the presence of concentrated hydrofluoric acid. See Diaz-Cabanas, M-J, Barrett, P. A., and Camblor, M. A. “Synthesis and Structure of Pure SiO2 Chabazite: the SiO2 Polymorph with the Lowest Framework Density”, Chem. Commun. 1881 (1998).
More recently, an aluminosilicate with the CHA framework type and having a silica to alumina molar ratio in excess of 100, such as from 150 to 2000, has been synthesized in the presence of fluoride ions. See U.S. Patent Application Publication No. 2003/0176751, published Sep. 18, 2003. Structure directing agents employed include N-alkyl-3-quinuclidinol, N,N,N-tri-alkyl-1-adamantammonium cations and N,N,N-trialkyl-exoaminonorbornane.
U.S. Published Patent Application No 2005/0154244, published Jul. 14, 2005, discloses a crystalline material comprising a CHA framework type molecular sieve with stacking faults or at least one intergrown phase of a CHA framework type molecular sieve and an AEI framework type molecular sieve, wherein the material is substantially free of framework phosphorus and has a composition involving the molar relationship (n)X2O3:YO2 wherein X is a trivalent element, Y is a tetravalent element and n is from 0 to about 0.5. The material can be synthesized using a mixed directing agent comprising an N,N,N-trialkyl-1-adamantylammonium compound and an N,N-diethyl-2,6-dimethylpiperidinium compound, normally in the presence of fluoride ions.
U.S. Published Patent Application No 2006/0115416, published Jun. 1, 2006, discloses a fluoride-free synthesis method for preparing a molecular sieve having the CHA crystal structure and a silica to alumina mole ratio of greater than 50:1, the method comprising: (a) forming an aqueous reaction mixture comprising a composition in terms of mole ratios falling within the following ranges:                YO2/WaOb 220-∞        OH−/YO2 0.19-0.52        Q/YO2 0.15-0.25        M2/nO/YO2 0.04-0.10        H2O/YO2 10-50wherein Y is silicon, germanium or mixtures thereof, W is aluminum, iron, titanium, gallium or mixtures thereof, a is 1 or 2, b is 2 when a is 1 or b is 3 when a is 2; M is an alkali metal or alkaline earth metal, n is the valence of M, and Q is a cation derived from 1-adamantamine, 3-quinuclidinol or 2-exo-aminonorbornane; and (b) maintaining said aqueous mixture under sufficient crystallization conditions until crystals are formed.        
U.S. Published Patent Application No 2008/0045767, published Feb. 21, 2008, discloses a method of synthesizing a crystalline material comprising a CHA framework type molecular sieve and having a composition involving the molar relationship:(n)X2O3:YO2 wherein X is a trivalent element; Y is a tetravalent element; and n is from 0 to less than 0.01, such as from about 0.0005 to about 0.007, the method comprising:
(a) preparing a reaction mixture capable of forming said material, said mixture comprising a source of water, a source of an oxide of a tetravalent element Y and optionally a source of an oxide of a trivalent element X, wherein the reaction mixture is substantially free of fluoride ions added as HF and further comprises an organic directing agent having the formula:[R1R2R3N—R4]+Q−wherein R1 and R2 are independently selected from hydrocarbyl alkyl groups and hydroxy-substituted hydrocarbyl groups having from 1 to 3 carbon atoms, provided that R1 and R2 may be joined to form a nitrogen-containing heterocyclic structure, R3 is an alkyl group having 2 to 4 carbon atoms and R4 is selected from a 4- to 8-membered cycloalkyl group, optionally substituted by 1 to 3 alkyl groups each having from 1 to 3 carbon atoms, and a 4- to 8-membered heterocyclic group having from 1 to 3 heteroatoms, said heterocyclic group being optionally substituted by 1 to 3 alkyl groups each having from 1 to 3 carbon atoms and the or each heteroatom in said heterocyclic group being selected from the group consisting of O, N, and S, or R3 and R4 are hydrocarbyl groups having from 1 to 3 carbon atoms joined to form a nitrogen-containing heterocyclic structure; and Q− is a anion;
(b) maintaining said reaction mixture under conditions sufficient to form crystals of said crystalline material; and
(c) recovering said crystalline material.
Because of the hazards inherent in working with HF, synthesis routes that will produce molecular sieves without the addition of fluoride ions are preferred. However, in the case of high silica CHA framework type materials, many of the synthesis routes that operate in the absence of HF produce molecular sieves that exhibit reduced OTO performance and particularly a high selectivity to coke as compared with the desired olefin products. There is significant interest in developing post-treatment methods for improving the OTO performance of CHA framework type and other small pore molecular sieves.
According to the present invention, it has now been found that the OTO performance of small pore size molecular sieves, such as high silica CHA framework type molecular sieves, and especially those produced by fluoride fee syntheses, can be enhanced by treatment with acids, particularly acetic acid, having a larger kinetic diameter than the pore diameter of the molecular sieve. In particular, it is found that the selectivity of the molecular sieves to undesirable coke and propane is reduced by the acid treatment, while the selectivity to ethylene and propylene is enhanced or substantially unaffected by the treatment. The desirable result is unexpected because NMR analysis suggests that there is no change in defect concentration or framework aluminum content as a result of the treatment. However, the improvement is significant and reproducible, indicating that the acid treatment is a promising method for improving the catalytic performance of fluoride-free high silica CHA materials. In contrast, it is found that treatment with acids, such as formic and hydrochloric acid, having a smaller kinetic diameter than acetic acid seems to lead to loss of crystallinity and reduced selectivity to ethylene and propylene.
In our co-pending United States Patent Application Publication No. 2007/0286798, published Dec. 13, 2007, we have described a process for improving the OTO performance of high silica CHA framework type molecular sieves, including those produced by fluoride free synthesis, by treating the molecular sieve with an atmosphere containing steam at a temperature of about 400° C. to about 650° C. for a time of about 8 hours to about 170 hours. The steaming is said to heal defects in the framework structure of the molecular sieve and to improve OTO performance by increasing the prime olefin selectivity of the molecular sieve.
Acetic acid treatment has been reported to heal framework defects in large pore (12-ring) molecular sieves. See, for example, Jones et al. “Synthesis of Hydrophobic Molecular Sieves by Hydrothermal Treatment with Acetic Acid”, Chemistry of Materials (2001), 13(3), pages 1041-1050. However, given the proposed mechanism in this article for the acetic acid treatment (dissolution of silica and transport of monomeric silicic acid-like species through the porous interior for insertion at the defect sites), similar results would not be expected with small pore zeolites, since the 8-ring pore opening is too small to allow silicic acid to be mobile within the pore system. In fact, no reports have been found of the use of acetic acid to treat small pore (8-ring) zeolites, such as CHA framework type materials.